1. Field of the Invention
The invention relates to a wheel bearing device which rotatably supports a wheel of an automobile on an car body.
2. Description of the Related Art
A wheel bearing device (hereinafter referred to as “bearing device”) shown in FIG. 45 is for a driving wheel and has a constitution in which a hub ring 1, a bearing 2, and a constant velocity universal joint 3 are unitized.
The hub ring 1 is provided with an outboard inner raceway 4 on its outer peripheral surface as well as a wheel mounting flange 6 for mounting a wheel (not shown). Hub bolts 7 for fixing a wheel disc are studded on the wheel mounting flange 6 with equal intervals in a circumferential direction. A pilot portion 40 having a wheel guide portion 38 and a brake guide portion 39 is unitizedly formed with an outboard end portion of the hub ring 1. Also, a brake rotor (not shown) is mounted on the wheel mounting flange 6 together with the wheel disc.
The constant velocity universal joint 3 is provided at an end of a shaft 8 and comprises a joint outer ring 11 with a track groove 9 formed on its inside periphery, a joint inner ring 12 having a track groove 10 formed on its outside periphery opposing the track groove 9 of the joint outer ring 11, torque transmission balls 13 installed between the track groove 9 of the joint outer ring 11 and the track groove 10 of the joint outer ring 11, and a cage 14 supporting the torque transmission balls 13 disposed between the joint outer ring 11 and the joint inner ring 12. Between the joint outer ring 11 and the shaft 8 is a boot 15 for preventing entry of water and foreign matter from outside and leakage of grease contained inside.
The joint outer ring 11 has a mouse portion 16 storing the joint inner ring 12, the torque transmission balls 13 and the cage 14, and a stem portion 19 axially extending from the mouse portion 16, with a serrated portion 17 being formed thereon. Transmission of torque from the stem portion 19 to the hub ring 1 is enabled by inserting the stem portion 19 into a through-hole of the hub ring 1 so that they are fit each other through serrated portions 17 and 18 formed respectively on an outside peripheral surface of the stem portion 19 and an inside peripheral surface of the through-hole. An axial end of the joint outer ring 11 is plastically deformed to be roll-formed to an outboard end portion of the hub ring 1, and the joint outer ring 11 is fixed to the hub ring 1 by a roll-formed portion 20.
Double-row inner raceways are formed with an outboard inner-raceway 4 formed on an outer peripheral surface of the hub ring 1 and with an inboard inner-raceway 5 formed on an outer peripheral surface of a shoulder portion 21 of the joint outer ring 11. The joint outer ring 11 is inserted into the hub ring 1 axially from the inboard side and is roll-formed to the hub ring 1. Thereby a shoulder portion 21 of the joint outer ring 11 butts against an inboard end portion of the hub ring 1, thereby pre-load is applied to the bearing 2.
The bearing 2 is of a double-row angular ball bearing constitution, and is constituted such that rolling elements 25 and 26 are disposed between the inner raceways 4 and 5 formed respectively on outer peripheral surfaces of the hub ring 1 and joint outer ring 11 and outer raceways 23 and 24 formed respectively on an outer ring 22, and the rolling elements 25 and 26 of each row are supported by cages 27 and 28 at equal intervals in a circumferential direction. The outer ring 22 is provided with an car-body mounting flange 29 for mounting an car body (not shown) on it. The car-body mounting flange 29 is fixed with bolts on a knuckle extending from a suspension device (not shown) of the car body. At opening portion at both ends of the bearing 2, a pair of seals 30 and 31 sealing an annular space formed by the outer ring 22, the hub ring 1 and the joint outer ring 11 are fitted into inner peripheral portions at end portions of the outer ring 22 to prevent inside grease from leaking and water and foreign matter from entering from outside. The seals 30 and 31 are provided with seal lips that are in sliding contact onto outer peripheral portions of the hub ring 1 and the joint outer ring 11.
While the bearing device shown in FIG. 45 is a type having a non-separable constitution in which the hub ring 1, the bearing 2 and the constant velocity universal joint 3 are unitized together, FIGS. 46 and 47 show other examples of bearing devices of a type having a separable constitution in which a hub ring 1′ and the bearing 2 are unitized together, and the constant velocity universal joint 3 is fixed to the hub ring 1′ with bolts 32 or nuts 33. A separable type differs from a non-separable type as described below.
An inner ring 35, a separate element from the hub ring 1′, is fitted onto a small-diameter end portion 34 formed on an outside periphery of an inboard end portion of the hub ring 1′, and an inboard inner raceway 5 is formed on an outside periphery of the inner ring 35. The inner ring 35 is pressed into position with an appropriate interference to prevent creeping from occurring. Both an outboard inner raceway 4 formed on an outside periphery of the hub ring 1′ and the inboard inner raceway 5 formed on the outside periphery of the inner ring 35 form double-row inner raceways. The inner ring 35 is pressed onto the end portion of the small-diameter portion 34 of the hub ring 1′, the end portion of the small-diameter portion 34 of the hub ring 1′ is outwardly roll-formed by plastically deforming it, and the roll-formed portion 36 serves to prevent the inner ring 35 from loosening and coming off, and to apply pre-load to the bearing 2.
In this type of bearing devices, because of the constitution in which the roll-formed portion 36 serves for the prevention of coming off and application of pre-load, the joint outer ring 11 is fixed to the hub ring 1 in the following way: a stem portion 19 of the joint outer ring 11 is inserted into the through-hole of the hub ring 1′, then the joint outer ring 11 is fixed to the hub ring 1′ with tightening torque necessary and sufficient to prevent the joint outer ring 11 from loosening from the hub ring 1′ either by engaging a bolt 32 into a threaded hole 37 formed on a stem portion 19 of the joint outer ring 11 (see FIG. 46) or by engaging a nut 33 onto the stem portion 19 of the outer joint ring 11 (see FIG. 47).
The bearing device in FIG. 45 is constituted such that the stem portion 19 of the joint outer ring 11 is roll-formed by plastically deforming it, and the joint outer ring 11 is fixed to the hub ring 1 by a roll-formed portion 20. Therefore, considering the convenience in assembly of this bearing device, a serration fit between the hub ring 1 and the stem portion 19 of the joint outer ring 11 is preferably loose.
Also, in the bearing devices in FIG. 46 and FIG. 47, the roll-formed portion 36 of the hub ring 1′ serves to prevent the inner ring 35 from loosening and coming off, and to apply pre-load to the bearing 2, and the bolt 32 or the nut 33 fixes the constant velocity universal joint 3 to the hub ring 1′. Because the swaged portion 36 of the hub ring 1′ serves to prevent the inner ring 35 from loosening and apply pre-load to the bearing 2 as described above, applying pre-load by fastening torque of the bolt 32 or the nut 33 becomes unnecessary, and the bolt 32 or the nut 33 fixes the joint outer ring 11 to the hub ring 1′ with fastening torque that is necessary and sufficient to prevent the joint outer ring 11 from loosening.
However, when the serration fit between the hub ring 1 or 1′ and the stem portion 19 is loose for convenience in assembly of the bearing device, play may occur between the serrated portions 17 and 18 of the hub ring 1 or 1′ and the stem portion 19 respectively, resulting in possible deterioration in drive feeling and generation of an unusual sound in a driving system. Also, a constitution of this type is weak to moment load applied to the hub ring 1, an attempt for securing sufficient strength for the stem portion 19 and the roll-formed portion 20 hinders the miniaturization of the entire device.
Also, because the bearing devices in FIGS. 45 to 47 are constituted such that the stem portion 19 of the joint outer ring 11 is fitted into an inside periphery of the hub ring 1 or 1′, the dimensions in a radial direction of the serrated portions 17 and 18 for torque transmission cannot be made larger than the inside diameter of the hub ring 1 or 1′. When the diameters of serrated portions cannot be made larger as described above, the serrated portions inevitably have to be made longer in an axial direction to secure a predetermined transmitted torque, which results in inconvenience in that the dimension in the axial direction of the bearing device increases.
In the bearing device in FIG. 45, specifically, moment load acting on the hub ring 1 is received mainly only by the outboard bearing 2 of the double row bearing 2. This is because the hub ring 1 and the joint outer ring 11 tend to be bent at their butted portion when moment load is applied, so that an inboard bearing is not capable enough to support moment load. Therefore, excessive force may cause looseness at the roll-formed portion 20, so that lack in coupling strength between the hub ring 1 and the outer joint ring 11, as well as lack in strength of the stem portion 19 of the joint outer ring 11 is feared.
Also, in the bearing devices in FIGS. 45 to 47, because the inner raceway 4 is formed on the hub ring 1 or 1′, and the hub ring 1 or 1′ has a function equivalent to a raceway of a general roller bearing, the hub ring 1 or 1′ must be made of steel for bearing in the same way as a raceway of such a general roller bearing. However, because steel for bearing is extremely purified steel, it is expensive. Also, steel for bearing contains a higher amount of carbon for hardenability and for hardness on a raceway surface, so that it has low ductility, which leads in poor workability in forging.
In a wheel bearing device, generally, an inboard bearing is subjected to severer load conditions than an outboard bearing. Conventionally, specifications of internal parts of both inboard and outboard bearings have been made equal without considering such a point described above. Briefly, the pitch circle diameters and other dimensions of outboard and inboard bearings have been made equivalent. This means, however, that an outboard outer raceway 23 also inevitably has a large radius dimension that satisfies the rated load of an inboard bearing, which is against demand for miniaturization and weight-reduction of a bearing device. Furthermore, when designing a bearing device, it must be considered that a hub bolts 7 do not contact with an outer ring 22 in case of repair, for example, in the case when the hub bolt 7 is drawn from wheel mounting flange 6; however, a larger radial dimension of the outboard outer raceway 23 makes such consideration difficult to realize, restricting the freedom in design. Consequently, the design itself of a bearing device cannot be realized in an extreme case.
In a bearing device in FIG. 45, a axial end of the joint outer ring 11 is plastically deformed to roll-form it to an outboard end portion of the hub ring 1, and the joint outer ring 11 is fixed to the hub ring 1 by the roll-formed portion 20. Therefore, during the roll-forming operation, run-out occurs on the wheel mounting flange 6 of the hub ring 1. Run-out of the flange will cause face run-out (or run-out in an axial direction) on a brake rotor to be mounted on the wheel mounting flange 6 of the hub ring 1, causing vibration in braking while the automobile is traveling at a high speed or resulting in a problem such as uneven wear of the brake rotor or brake juddering.
Generally, in view of the reason of easiness in forging, workability in cold forging, and machinability or because of economy, raw un-heat-treated medium carbon steel (S53C and others) for machine structural purpose is used for the hub ring 1. Miniaturization and weight-lightening of a bearing device greatly contribute for increased travel stability of an automobile, so that the wheel mounting flange 6 of the hub ring 1 is increasingly constituted with ribs and thinned. However, such movement is bringing the mechanical strength of the hub ring itself to a fatigue limit of the material, or medium-carbon steel for machine structural purpose, and therefore, further weight-reduction is becoming difficult. Specifically, thinning of the wheel mounting flange 6 of the hub ring 1 for weight-reduction purpose can cause concentration of rotational bending stress at an outboard base portion of the wheel mounting flange 6, or at a fillet located at a region extending from a brake rotor mounting surface to a cylindrical pilot portion 41, and the fillet can be a starting point of breakage.
An inboard base portion of the wheel mounting flange 6 is a sealing surface with which the seal-lip of a seal 30 is in sliding contact, and the sealing surface has a larger curvature with a quenching-tempering treatment being applied to give abrasion resistance on the surface. Therefore, the inboard root portion of the wheel mounting flange 6 has higher mechanical strength than an outboard base portion that is not heat-treated, so that the inboard base portion is less likely to be a starting point of breakage caused by rotational bending stress.
Although thickening of the wheel mounting flange 6 can be a solution for avoiding such breakage described above, it is against the movement of weight-reduction. Further, generation of stress can be eased with enlarged dimensions of the base portion, or the curvature of the base portion, of the wheel mounting flange 6; however, the application of this method is limited by a possible mechanical interference between the base portion and a brake rotor to be mounted on the wheel mounting flange 6.
Also, material can be strengthened by increasing its carbon content, adding a strengthening element such as Si (silicon) or V (vanadium) or by applying a heat treatment such as normalizing; however, workability of the material is affected by increased material hardness. Consequently, conventional processing methods or existing facilities become difficult to be applied, and further, adding a large amount of strengthening elements leads to increased material costs.